Canadian Patent Number 782,203 discloses apparatus for irregular stretching yarns. The apparatus consists of a rotating wheel with an eccentrically mounted pin thereon. The pin engages the yarn during the travel of the yarn over the wheel, and lengthens and shortens the path of travel of the yarn during wheel rotation.
French Patent Number 1546531, corresponding to United States patent application Ser. No. 739,519 now U.S. Pat. No. 3,611,521 discloses apparatus generally similar to the above Canadian patent but with a particular type of yarn engaging pin so arranged that the yarn is subjected to sudden disengagement from the pin.
Polybutylene terephthalate polymers are known to the art. See, for example, U.S. Pat. No. 2,465,319, the disclosure of which is hereby included by reference to the extent necessary to understand the polybutylene terephthalate polymers disclosed therein and the processes for making same. French patent 2026544 discloses paint brush bristles made of polybutylene terephthalate which unlike polyethylene terephthalate, can be worked into filaments, branched, and pointed to form satisfactory paint brush bristles.
Yarns having irregular diameters are highly desired in the production of fancy textile articles as they impart a particular desired appearance. Slubbed or knop yarns may be mentioned among irregular diameter yarns which produce fancy effects. Such yarns may be produced by certain twisting operations, or by assembling the yarns in certain manners.
Some processes have been developed to produce irregular diameter, single or multi-filament, synthetic yarns, with such process generally involving modifications of the spinning or drawing processes.
Irregular diameter polyamide yarns that are produced by the art cannot be used for certain important applications, such as for dress trimmings, because of low resistance to light. This low light resistance of polyamide yarns is characterized, among other things, by yellowing and chemical degradation of the mechanical properties. Polyester yarns, especially polyethylene terephthalate, have good photochemical degradation resistance and are suited for certain applications, in particular for the production of irregular diameter yarns whose crystallinities differ in the thick portions and the thin portions, whereby appreciable differences in color intensity are obtained when such yarn is dyed. However, such polyester yarns exhibit a major drawback in that after thermal treatments, such as dying and/or steaming, the yarn will have a harsh feel, which is undesired for most textile applications, because of differences of orientation which are characterized by isotropic crystallization in the thick portions, which also have mediocre mechanical properties. Polyethylene terephthalate yarns have an average Young's modulus, or modulus of elasticity, in the order of 900 to 1400 g/tex.